Conventional time domain multiple access (TDMA) satellite communication networks employ multiple radio stations which communicate through an earth satellite repeater by transmitting time-synchronized bursts of radio energy relative to the repeater and which receive a time multiplex composite of bursts containing corresponding modulated information from the repeater. In TDMA operations, multiple ground stations associated with radio signaling nodes transmit bursts of time-concentrated information signals on a shared carrier frequency spectrum and receive the same information signals after repetition by the satellite repeater on a shifted carrier frequency spectrum. Each ground station is assigned a particular time slot in a continuum of recurrent frames for transmission of its bursts and for the reception of its own bursts and the bursts of other stations. The bursts interleave at the satellite in close time formation without overlapping.
Each earth station includes connections to incoming digital lines and voice lines originating from terrestrial sources. These input lines are respectively connected to digital data ports and voice ports on a satellite communications controller (SCC) at the station. The SCC is a computer controlled satellite communications switching system which employs digital transmission techniques in a time division multiple access format. The TDMA format divides the time at the satellite into fixed length units called frames. Each earth station in a transponder is assigned a portion of the frame in which to transmit its traffic burst. Each traffic burst is received by all earth stations in the transponder. The amount of time assigned for each earth station's traffic burst may be different for each earth station and may also vary over time. A frame consists of a fixed time period allocated for transmission of network control and synchronization information and for transmission of traffic from the active earth stations in the network to one or more other earth stations in the network.
To make the most efficient use of the TDMA frame for a satelite communications network, the assignment of each local earth station's TDMA burst duration can be based on a statistical assessment of the demand by local voice and data ports, as is described in the copending U.S. patent application Ser. No. 079,928 filed Sept. 28, 1979 by J. W. Fennel, Jr., et al. entitled "Demand Assignment Technique for a TDMA Satellite Communication Network," assigned to the instant assignee. occasionally the actual voice and data port demand at a local station exceeds that station's assigned burst duration so that some of the messages which are ready for transmission cannnot be transmitted and may be frozen-out. For example, U.S. Pat. No. 4,009,343 to Markey, et al., discloses the storage of digitized message increments in a satellite communications controller, for a retransmission over a satellite link. If the input load from terrestrial sources to a local station exceeds the capacity of the traffic burst assigned to that station, part of the traffic load will be frozen-out. Traffic to be canceled is selected in a predetermined order of priority favoring data over voice activity and continuing voice talkspurt activity over initial talkspurt activity. However the disclosed system is limited to requiring that every port have a fixed virtual channel position in the TDMA burst. Only by the use of a voice activity compression (VAC) mask, can the actual transmission of a given channel be omitted when total channel activity has increased. What is needed is a more flexible means for ordering the priority of messages to be transmitted in a TDMA burst.